JP2003293022A - Method for producing molten low nitrogen and chromium- containing steel - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method for producing molten low nitrogen and chromium-containing steel by which molten low nitrogen and high purity chromium-containing steel is provided by reducing a degassing treatment in a reduced-pressure secondary decarburization-refining process and the shortening of a treating time in a reduced-pressure secondary decarburization-refining furnace, the reduction of a refractory cost, etc., can be obtained. <P>SOLUTION: A reduction-treatment of chromium oxide in slag is applied by dividedly adding CO<SB>2</SB>generating material 13 after adding a reduced agent into the molten chromium-containing steel performing the decarburization- refining in a converter 1. Successively, the reduced-pressure secondary decarburization-refining is applied after tapping off the chromium-containing steel 8 performing the reduction treatment in a ladle 12 charging the CO<SB>2</SB>generating material 13. <P>COPYRIGHT: (C)2004,JPO

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing a low nitrogen chromium-containing molten steel, which comprises melting a chromium-containing molten steel having a low nitrogen content using a converter and a reduced pressure secondary decarburizing / refining furnace.

[0002]

2. Description of the Related Art Conventionally, molten chromium-containing steel is decarburized to a carbon concentration of 0.3 to 1.0% by mass using a converter-type decarburizing and refining furnace such as a top-bottom blowing converter and a top-blowing converter. After melting the above-mentioned crude molten steel, the crude molten steel is tapped in a ladle. Then, in the decompression secondary decarburization refining process using this ladle, after adding an Al alloy for heating, the mixture is blown with acid to raise the temperature of the crude molten steel and perform the secondary decarburization refining, and then the chromium-containing molten steel. Is melted. Furthermore, in recent years,
In order to improve the workability of chromium-containing molten steel, the number of steel types aiming at low nitrogen (low N) is increasing, and although low N is being performed in the converter, it is not sufficient, and the decompression secondary decarburization refining treatment is not sufficient. Degassing is forced to be performed during the process of blowing acid decarburization refining, which causes problems such as extended processing time of the decompression secondary decarburization furnace, increased refractory costs, and increased energy consumption. Is occurring.

As a countermeasure against this, Japanese Patent Laid-Open No. 58-18931
As described in Japanese Patent Publication No. 5, a tap hole opening / closing device is provided at the tap hole of the converter, a communicating pipe is attached to the tap hole opening / closing device, and the tip of the communicating pipe is inserted into the ladle. In addition,
It has been proposed to cover the surface of the ladle with a lid and blow argon gas into the interior to produce low N and low hydrogen (H) molten steel. Further, as described in JP-A-58-189315, when Ca alloy is added to molten steel in a ladle equipped with a lid, boil is generated due to strong reaction of Ca alloy and air and molten steel are separated from each other. In order to prevent the nitrogen in the air from coming into contact with the slag, a CO ₂ -containing substance such as calcium carbonate or dolomite is added to the slag inside the ladle to create a CO ₂ atmosphere inside the ladle and inside the slag. , It is carried out to prevent the molten steel in the ladle from being absorbed.

[0004]

However, the method described in Japanese Patent Laid-Open No. 58-189315 requires a tap hole opening / closing device, a communicating pipe, a pot lid, and other devices at the tap hole of the converter. However, the equipment becomes large-scale, the equipment cost increases, or it becomes difficult to monitor the tapping operation from the outside, and it is not easy to confirm the tapping completion. As a result, it may not be possible to suppress the converter slag that flows into the ladle, causing a situation such as chromium loss due to the converter slag that has flowed in during the depressurization secondary decarburization and slag excess overflow or bumping, The operation becomes unstable. Further, in the method described in Japanese Patent Laid-Open No. 58-189315, it is possible to suppress the absorption of molten steel in ladle refining, but the absorption of nitrogen-containing molten steel in the reducing treatment process of the converter or during tapping. You cannot prevent nitriding.

As a result, it is necessary to perform degassing treatment for denitrification during the depressurization secondary decarburization refining. In addition, if the reduction treatment in the converter and the absorption of nitrogen during tapping cannot be suppressed, the nitrogen (N) of the product becomes high, and it is impossible to produce high-purity molten steel excellent in workability and the like. As described above, Japanese Patent Laid-Open No. 58-189
As in the method described in Japanese Patent No. 315 gazette, even if nitrogen absorption in the ladle refining process is suppressed, in order to produce low-purity high-purity molten steel, denitrification is performed during depressurization secondary decarburization refining. The degassing process cannot be avoided, and there are problems that the processing time of the depressurized secondary decarburization and refining furnace is increased, the refractory cost is increased, and the energy consumption is increased.

The present invention has been made in view of the above circumstances. The degassing process in the depressurization secondary decarburizing and refining process is reduced to produce a high-purity chromium-containing molten steel with low nitrogen, and the secondary degassing under reduced pressure is performed. An object of the present invention is to provide a method for producing a low nitrogen chromium-containing molten steel capable of shortening the processing time of a coal refining furnace and reducing the cost of refractory materials.

[0007]

The method for producing a low-nitrogen chromium-containing molten steel according to the present invention in accordance with the above-mentioned object is to reduce CO ₂ after adding a reducing agent to the chromium-containing molten steel which has been decarburized and refined in a converter.
The generated substance is dividedly added to reduce the chromium oxide in the slag, and then the reduced chromium-containing molten steel is tapped into a ladle containing the CO ₂ generating substance, and the secondary pressure reduction is performed. Perform decarburization refining. By this method, decarburization refining in the converter is performed, and when the carbon concentration reaches 0.3 to 1.0% by mass, the reducing agent is added. Therefore, the decarburizing slag containing chromium oxide by the reducing agent is added. Promotes the reduction reaction of
By dividing and adding the CO ₂ -generating substance, the surface of the crude molten steel exposed by the decomposed CO ₂ gas can be covered during the reduction treatment period, and the nitrogen absorption of the crude molten steel can be suppressed.

Depending on the CO ₂ generating substance, for example,
When calcium carbonate is used as the CO ₂ generating substance and added all at once, the endothermic heat of the CO ₂ generating substance deteriorates the fluidity of the slag and slows down the reduction rate of chromium oxide, and the surface of the molten chromium-containing steel thereafter. Of CO ₂ gas also deteriorates the coverage of the molten chromium-containing steel, resulting in severe absorption of nitrogen. On the other hand, when the CO ₂ -generating substance is added in two or more parts, the rapid endothermic absorption of the CO ₂ -generating substance is suppressed and the reduction rate of chromium oxide is excellent without causing an extreme decrease in the fluidity of the slag. In addition, the surface of the molten chromium-containing steel can be continuously covered with CO ₂ gas to suppress nitrogen absorption of the molten chromium-containing steel.

Furthermore, at the time of tapping (when the furnace is tilted), the inside of the furnace is C
Since it is not possible to create an atmosphere of O ₂ gas, it is necessary to make the slag in the furnace in a molten state and cover the molten chromium-containing steel with molten slag in order to prevent nitrification of the molten chromium-containing steel from the air entering the furnace. In order to stably satisfy this condition, the basicity (CaO / SiO ₂ ) of the slag is 1.2 to 1.7.
It is preferable to set it to 1.3 to 1.4. Further, since the atmosphere in the ladle during tapping and during tapping is a CO ₂ gas atmosphere, it is possible to suppress the contact of the crude molten steel with air, and to produce the low nitrogen (N) crude molten steel. You can

In the method for producing a low-nitrogen chromium-containing molten steel according to the present invention, a slag melting point reducing agent is added to the chromium-containing molten steel in the ladle containing the CO ₂ generating substance. As a result, while preventing the molten chromium-containing steel from contacting the air with the CO ₂ gas generated from the CO ₂ -generating substance, the molten chromium-containing steel in the ladle is covered with the molten slag by the slag lowering agent, and the molten chromium-containing steel is obtained. It is possible to suppress nitrogen absorption from the surface.

In the method for producing a low nitrogen chromium-containing molten steel according to the present invention, the CO ₂ generating substance is calcium carbonate, and the slag lowering agent is produced by using a flux of SiO ₂ .Al ₂ O ₃ system. It is advisable to adjust the ratio of the SiO ₂ and Al ₂ O ₃ concentrations to the CaO concentration to be 1.3 to 1.8. As a result, the slag formed on the molten steel containing chromium in the ladle can be stably melted, and the molten slag can be sufficiently covered. Further, it is possible to stably suppress nitrogen absorption from the surface of the chromium-containing molten steel. The ratio of the concentration of SiO ₂ and Al ₂ O _{3 to the} concentration of CaO generated (CaO / SiO ₂ +
When Al ₂ O ₃ ) is less than 1.3 or exceeds 1.8, the fluidity of the slag is lowered, and the effect of preventing nitrogen absorption of the molten chromium-containing steel in the secondary decarburization refining from tapping is deteriorated.

In the method for producing a low nitrogen chromium-containing molten steel according to the present invention, an Al alloy for heating is added to the chromium-containing molten steel in the reduced pressure secondary decarburization refining. This makes it possible to quickly raise the temperature of the crude molten steel and suppress the oxidation of chromium in the reduced pressure secondary decarburization refining. Further, in the method for producing a low nitrogen chromium-containing molten steel according to the present invention, the CO ₂ generating substance is calcium carbonate, and CaO produced by reacting with the Al alloy for heat raising added in the reduced pressure secondary decarburization refining
It is advisable to adjust the ratio of Al ₂ O ₃ to 1.2 to 1.6. As a result, Al ₂ generated by using for heating
The remaining CaO that has generated CO ₂ from O ₃ can improve the fluidity of the depressurized secondary decarburizing slag and can suppress Al ₂ O ₃ -based inclusions mixed in the molten chromium-containing steel.

The ratio of CaO to Al ₂ O ₃ (CaO / Al ₂
When O ₃ ) is less than 1.2, the fluidity of the depressurized secondary decarburization slag becomes too good, and the wear of the refractory in the ladle increases, and the inclusion of Al ₂ O ₃ -based inclusions in the molten steel also increases. However, the quality of molten steel containing chromium deteriorates. On the other hand, CaO and Al ₂ O ₃
If the ratio exceeds 1.6, the fluidity of the depressurized secondary decarburizing slag becomes poor, and it takes time to reduce the chromium oxide in the generated secondary decarburizing slag. Cause an increase. In addition, in the method for producing a low nitrogen chromium-containing molten steel according to the present invention, it is preferable to seal the outer periphery of the porous plug provided at the bottom of the ladle with an argon gas atmosphere. As a result, the air that enters the molten steel through the joints of the porous plug, which is necessary for stirring the inside of the ladle in the secondary decarburization under reduced pressure, can be blocked, and the absorption of nitrogen in the molten steel can be prevented.

[0014]

BEST MODE FOR CARRYING OUT THE INVENTION Next, referring to the attached drawings, an embodiment in which the present invention is embodied will be described to provide an understanding of the present invention. FIG. 1 is an explanatory view of an upper-bottom blowing converter applied to a method for producing a low-nitrogen chromium-containing molten steel according to an embodiment of the present invention, and FIG. 2 is an explanatory view of tapping of the upper-bottom blowing converter. Is an explanatory view of a sealing structure of a porous plug provided at the bottom of the ladle, FIG. 4 is a graph showing the relationship between the total CaO amount and the N value of the ladle, and FIG. 5 is a graph showing the secondary blow occurrence frequency.

As shown in FIGS. 1 and 2, an upper-bottom blown converter 1 which is an example of a converter used in a method for producing a low nitrogen chromium-containing molten steel according to an embodiment of the present invention comprises a furnace body 2 Bottom blowing nozzle 3 for blowing agitation or a gas that also serves as agitation and fuel to the bottom of the furnace, a tapping port 4 on the upper side of the furnace body 2, hot metal, scrap iron, ferroalloy (Fe-Cr), quick lime, A charging port 5 for charging a raw material such as calcium carbonate is provided. Furthermore,
Above the charging port 5, a lance 6 which is held so as to be vertically movable and blows oxygen, and an auxiliary material hopper 7 for feeding quicklime, a reducing agent, and the like into the furnace body 2 are provided. Also, the upper and lower blowing converter 1
Porous plug 11 for inclining the furnace body 2 and supplying argon gas for stirring to the bottom portion for receiving the crude molten steel 8 from the tapping port 4
CO ₂ attached to the ladle 12 and above the ladle 12
A storage hopper 14 for storing calcium carbonate 13, which is an example of a gas generating substance, is provided.

FIG. 3 is an explanatory view of the sealing structure of the porous plug provided at the bottom of the ladle. FIG. 3A is a general description, and FIG. 3B is an enlarged view of the porous plug portion. As shown in FIGS. 3 (a) and 3 (b), the porous plug 11 attached to the bottom portion of the ladle 12 is provided with a lining refractory material 15 through an insertion hole of the porous plug 11 formed in the ladle iron shell 20. It is attached through the joint 16. Below the porous plug 11, there is provided a ring 19 having a hollow interior with a plurality of holes 18 formed between a disk-shaped seal iron plate 17a and a seal iron plate 17b.
Is in contact with the ladle iron skin 20 so that there is no leak. The argon gas is supplied to the porous plug 11 from an argon gas pipe 21, and the ring 19 is supplied from a seal gas pipe 22. Reference numeral 9 is a converter slag when the decarburization refining is completed, and reference numeral 10 is a slag after the reduction treatment is completed and calcium carbonate is added.

Next, a method for manufacturing the low nitrogen chromium-containing molten steel according to the present embodiment using the upper-bottom blowing converter 1 will be described. Blast acid for charging hot metal, scrap iron, quicklime, etc. into the furnace body 2 of the top-bottom blowing converter 1 from the charging port 5 and lowering the lance 6 while blowing a gas for stirring from the bottom-blowing nozzle 3 to blow oxygen Started. After the temperature inside the furnace body 2 reached 1500 ° C or higher, the Fe-Cr alloy was added, and then the lance 6 was added.
The decarburization refining was performed by blowing in the blowing acid from No. 3 and the stirring gas from the bottom blowing nozzle 3. And the carbon concentration is 0.3
At the time when the content reached ˜1.0% by mass, the blowing acid was stopped and the lance 6 was raised to complete the decarburization refining. Since the converter slag 9 at this time contains a large amount of chromium oxide (Cr ₂ O ₃ ),
Since the fluidity is poor and the surface of the crude molten steel 8 is not sufficiently covered, air enters the furnace body 2 at the same time as the end of the blowing acid, and the surface of the crude molten steel 8 is agitated by the gas for stirring from the bottom blowing nozzle 3. Exposed and in contact with air.

Therefore, the Fe-Si alloy, which is an example of a reducing agent, is added to the converter slag 9 to complete the dispersion and dissolution of the Fe-Si alloy, that is, 0.3 to 0.3% after the addition of the Fe-Si alloy. Promptly after 1.5 minutes calcium carbonate (CaC
Cut out the O ₃₎ from Fukuzai hopper 0.5 to 1.0 t /
Add charge. The addition of CaCO ₃ fills the furnace body 2 with the thermally decomposed CO ₂ gas, suppresses the intrusion of air into the furnace, and prevents the surface of the crude molten steel 8 from coming into contact with air. However, within the range of 1.6 to 3.0 minutes from the first addition of CaCO ₃ , the second addition of CaCO _{3 is} added at 0.5 to 1.0 ton / charge. By this divided addition, the inside of the furnace body 2 is replaced with CO ₂ gas, and a large amount of Cr _{2 is} added.
Prevents nitrogen absorption of crude molten steel 8 by reliably suppressing contact of the surface of crude molten steel 8 with air in the early to middle stages of the reduction treatment that contains O ₃ and cannot express a sufficient slag cover effect with poor fluidity. can do.

Further, the above-mentioned CaCO₃The addition of
In addition, the converter slag 9 has low fluidity due to the rapid heat absorption.
It is possible to suppress lowering, and moreover, converter slag 9
Has a basicity of 1.2 to 1.7, preferably 1.3 to 1.4
Is adjusted so that the converter slag 9 is melted.
Can improve the fluidity in the
The bar effect can be sufficiently expressed. Like this
Then, after performing the reduction treatment for 4 to 6 minutes, the furnace body 2 is tilted.
The crude molten steel 8 is tapped into the ladle 12 from the tapping port 4. This ladle
Ladle from the storage hopper 14 at the time of tapping the crude molten steel 8 to the 12
Calcium carbonate (CaCO₃) From 0.1 to 0.
5 ton / pot and SiO which is an example of a slag melting point reducing agent₂
・ Al₂O₃Made of a system such as SiO₂Is 38% by mass,
Al₂O ₃Is composed of 25 mass% and the balance is CaO.
Put 100-150 kg / pot in a pot. Change
The porous plug 11 on the bottom of the ladle 12
Blow-in of argon gas supplied from Lugon gas pipe 21
Do the hot water immediately after receiving the hot water.

CaCO placed in the ladle 12₃Is
Due to the heat of the crude molten steel 8, CaO and CO ₂Cause a decomposition reaction
CO generated₂CO in the ladle 12₂In the atmosphere
It is possible to prevent air from entering and reduce the nitrogen content of the crude molten steel 8.
Can be achieved. Moreover, the added CaCO₃Minutes
CaO after unraveling reacts quickly with the slag lowering agent
The molten slag can be generated. Also this
CaCO₃Of CaO concentration after decomposition of SiO₂
+ Al₂O₃Concentration ratio [CaO / (SiO₂+ Al₂O
₃)] Is 1.3 to 1.8 so that the melting point of the slag is lowered.
Adjusting the amount of agent added, or CaO or SiO
₂And Al₂O₃To meet the above concentration ratio
Can be adjusted.

Since sufficient cover slag for covering the surface of the crude molten steel 8 can be formed, direct contact of the surface of the crude molten steel 8 with air can be avoided, and the ladle 12 can be used.
From the lower part of the furnace body 2 to a decompression secondary decarburization refining device (not shown)
It is possible to prevent air from coming into contact with the crude molten steel 8 due to boil of argon gas blown from the porous plug 11 in the process of transporting to. Furthermore, ladle 12
At the bottom of the porous plug 11 attached to the lining refractory 15 through the joint 16 from the insertion hole formed in the ladle iron skin 20 of the porous plug 11 provided at the bottom of the disk, a disk-shaped seal iron plate 17a and a seal are provided. Argon gas is supplied from a seal gas pipe 22 to a hollow ring 19 having a plurality of holes 18 between the iron plates 17b, and the porous plug 18 is supplied by the argon gas blown from the plurality of holes 18 of the hollow ring 19.
The outside air (air) entering through the gap between the joint 16 of the refractory lining 15 and the joint 16 is blocked.

Further, the crude molten steel 8 tapped from the furnace body 2 is
In order to compensate for the temperature, metal Al, which is an example of a heating agent, is added.
After adding and heating, decompression secondary decarburization refining is performed. Only
Al in the slag generated by the combustion of metallic Al₂O₃
When the concentration of Al increases, Al becomes₂
O₃Inclusions in the system increase and impair the quality of cast and steel products
It will be. So, put it in the ladle 12 and disassemble it.
Effectively utilizing the generated CaO for decompression secondary decarburization refining
Al produced by combustion of added metallic Al ₂O₃of
Concentration value is CaO / Al₂O₃Becomes 1.2 to 1.6
As described above, adjustment is performed in consideration of addition of metallic Al, quick lime, and the like.

As a result, the fluidity of the depressurized secondary decarburizing slag is improved, the Al ₂ O ₃ -based inclusions mixed in the molten chromium-containing steel can be suppressed, and the chromium oxidation in the secondary decarburizing slag is suppressed. The reduction treatment of the substance can be performed promptly, and the reduction treatment time and the use of the reducing agent can be reduced. And
As shown in FIG. 4, the nitrogen concentration of the conventional Cr-containing crude molten steel to which CaCO ₃ is not added during the reduction treatment has a nitrogen concentration of 100 to 200 p.
In contrast to the high pm, in the case of the present embodiment, the extremely low nitrogen chromium-containing crude molten steel 8 having a nitrogen concentration of 40 to 90 ppm can be smelted, and the secondary decarburization is performed by using a reduced pressure secondary decarburization refining device. By performing refining, it was possible to produce an extremely low nitrogen chromium-containing molten steel having a nitrogen concentration of 40 to 90 ppm. It is possible to improve the workability and other qualities of the steel material processed from this slab.

[0024]

[Examples] Next, implementation of a method for producing low nitrogen chromium-containing molten steel
An example will be described. From the charging port into the furnace of the top-bottom blow converter
Hot metal (30 tons) of hot metal and scrap iron are charged, and a hopper for auxiliary material
Fe / Cr alloy, quicklime and other auxiliary materials are charged into the furnace body
While blowing a stirring gas from the bottom blowing nozzle provided
Lower the lance and add oxygen to 28,000 Nm ³/ Hour of acid
Starts blowing acid at a high speed and raises the temperature in the furnace to 1500 ° C or higher.
After that, add Fe-Cr alloy and continue
And the blowing gas from the bottom blowing nozzle.
Decarburization until the carbon concentration reaches 0.3 to 1.0 mass%
Ren finished.

After the decarburization refining is completed, the Fe-Si alloy is added into the furnace, and after 0.3 to 1.5 minutes, calcium carbonate (CaCO ₃ ) is cut out from the auxiliary material hopper and 0.5 to
Add 1.0 ton / charge, and then the first CaC
2nd CaC within 1.6-3.0 minutes after addition of O ₃
O _{3 was} added in an amount of 0.5 to 1.0 ton / charge to reduce the slag in the converter, and Cr ₂ O ₃ in the converter slag was recovered as Cr in the crude molten steel. The crude molten steel that has undergone the reduction treatment is transferred from the storage hopper to a calcium carbonate (Ca
CO ₃ ) 0.1 to 0.5 ton / pot and SiO ₂ · Al
_The steel was tapped into a ladle containing 100 kg of a ₂ O ₃ -based slag lowering agent. This crude molten steel was subjected to decarburization refining of fumaric acid by raising the temperature by adding metal Al using a decompression secondary decarburization refining device such as VOD. Further, in the ladle, argon gas was blown at 400 to 600 Nm ³ / hour from a porous plug provided at the bottom until continuous casting.

In Example 1, Fe reduction was performed during the reduction treatment in the converter.
1.0 minute after the addition of the Si alloy, 0.5 ton / charge of calcium carbonate was added, and
2.0 minutes after the second CaCO ₃ addition, the second Ca
Add 0.5 ton of CO ₃ / charge to reduce converter slag and add 0.3 ton of calcium carbonate to ladle
This is the case where a pot and 100 kg of the slag lowering agent are placed and then the secondary decarburization refining under reduced pressure is performed, and the nitrogen concentration is 60 p.
It is possible to produce pm chromium-containing molten steel, and as shown in FIG. 5, the occurrence frequency of secondary blow (when degassing treatment is performed during the reduced pressure decarburization refining) in the reduced pressure secondary decarburization refining is 0. .09
And was good.

In Example 2, Fe during Fe reduction treatment was used.
1.0 minute after the addition of the Si alloy, 0.5 ton / charge of calcium carbonate was added, and
2.0 minutes after the second CaCO ₃ addition, the second Ca
Add 1.0 ton of CO ₃ / charge to reduce converter slag and add 0.3 ton of calcium carbonate to ladle
This is a case where a pot and 100 kg of a slag melting point reducing agent are placed, after which secondary vacuum decarburization and refining are performed, and the invasion of air from the joint part of the porous plug at the bottom of the ladle is sealed with argon gas, the nitrogen concentration It was possible to smelt a chromium-containing molten steel of 40 ppm, and as shown in FIG. 5, the occurrence frequency of secondary blow in the reduced pressure secondary decarburization refining was 0.07, which was extremely good.

In Example 3, during the reduction treatment in the converter, Fe
1.0 minute after the addition of the Si alloy, 0.5 ton / charge of calcium carbonate was added, and
2.0 minutes after the second CaCO ₃ addition, the second Ca
Add 0.5 ton of CO ₃ / charge to reduce converter slag and add 0.3 ton of calcium carbonate to ladle
A pot and 100 kg of the slag deliquescent agent are placed, and then CaO / Al ₂ O ₃ of the secondary decarburization under reduced pressure is added to 1.2 to 1.5.
It was possible to produce molten chromium-containing steel having a nitrogen concentration of 60 ppm, and it was possible to prevent defects of Al ₂ O ₃ -based inclusions in the molten chromium-containing steel.

On the other hand, during the reduction treatment in the converter, Fe-
This is a case where Si alloy was added, calcium carbonate was not added during the reduction treatment, and calcium carbonate was not placed in the ladle. As shown in FIG. Extremely bad at 0.20, nitrogen concentration 150p
It was as high as pm. Although the embodiments of the present invention have been described above, the present invention is not limited to the above-described embodiments, and all modifications and the like of the conditions without departing from the gist are within the scope of the present invention. For example, as the CO ₂ generating substance, in addition to calcium carbonate, magnesium carbonate or a carbonate compound such as raw dolomite can be used. Further, calcium carbonate can be continuously added after the reducing agent is added in the converter, and the surface of the crude molten steel is always covered with CO _2, which is preferable.

[0030]

As described above, according to the present invention, the reducing agent is added to the decarburized and smelted chromium-containing steel in the converter, and then the CO ₂ -generating substance is dividedly added to the chromium oxide in the slag. Is performed, and then the reduced-pressure secondary decarburization refining process is performed after tapping the reduced chromium-containing molten steel in the ladle containing the CO ₂ -generating substance. It is possible to reduce the degassing treatment in step 1 to produce low-nitrogen high-purity molten steel, and shorten the processing time of the reduced pressure secondary decarburizing and refining furnace and the cost of refractory materials.

[0031] In particular, the production method of low nitrogen containing chromium molten steel according to claim 2, since the addition of slag low melting agent to chromium-containing molten steel in the ladle containing the CO ₂ generating material, the CO ₂ generating material The synergistic effect of covering the surface of the molten chromium-containing steel in the ladle with the generated CO ₂ gas and the slag melted by the slag melting point reducing agent can suppress the absorption of nitrogen from the surface of the molten chromium-containing steel.

Further, in the method for producing a low nitrogen chromium-containing molten steel according to claim 3, the CO ₂ generating substance is calcium carbonate,
As the slag lowering agent, a flux composed of a SiO ₂ · Al ₂ O ₃ system is used, and Si for the generated CaO concentration is used.
Since the ratio of O ₂ and Al ₂ O ₃ concentration is adjusted to 1.3 to 1.8, the slag formed on the molten chromium-containing steel in the ladle is stably melted, and the molten slag is used. The surface of the chromium-containing molten steel can be sufficiently covered and the nitrogen absorption from the surface of the chromium-containing molten steel can be stably suppressed.

Further, in the method for producing a low nitrogen chromium-containing molten steel according to claim 4, since the Al alloy for heating is added to the chromium-containing molten steel in the reduced pressure secondary decarburization refining, the heating of the crude molten steel is rapidly carried out. It is possible to suppress the oxidation of chromium in the secondary decarburization and refining under reduced pressure, and to reduce the reducing agent. Further, the method for producing a low nitrogen chromium-containing molten steel according to claim 5,
The CO ₂ generating substance is calcium carbonate, and C generated by reacting with the Al alloy for heat raising added in the secondary decarburization under reduced pressure
Since the ratio of aO to Al ₂ O ₃ is adjusted to 1.2 to 1.6, the high melting point of slag due to Al ₂ O ₃ used for heating is suppressed to suppress the secondary decompression of the slag. Improves the fluidity of the charcoal slag, promotes the decarburization reaction and the reduction of chromium oxide, suppresses the wear of the refractory of the ladle, and causes the defects of cast slabs and steels caused by Al ₂ O ₃ inclusions. Can be prevented and the quality of the cast slab and the steel material can be improved.

Further, according to the method for producing low-nitrogen chromium-containing molten steel according to claim 6, since the outer periphery of the porous plug provided at the bottom of the ladle is sealed with an argon gas atmosphere, a secondary pressure reduction from the steel output from the converter is performed. It is possible to prevent nitrification of molten steel from the air that enters the molten steel from the joints of the porous plug during decarburization refining, and it is possible to manufacture chromium-containing molten steel with extremely low nitrogen.

[Brief description of drawings]

FIG. 1 is an explanatory view of an upper-bottom blowing converter applied to a method for producing a low-nitrogen chromium-containing molten steel according to an embodiment of the present invention.

FIG. 2 is an explanatory view of a tapped steel of the upper bottom blowing converter.

FIG. 3 is an explanatory diagram of a seal configuration of a porous plug provided at the bottom of a ladle.

FIG. 4 is a graph showing the relationship between the total CaO amount and the ladle N value.

FIG. 5 is a graph showing a secondary blow occurrence frequency.

[Explanation of symbols]

1 Top-bottom blowing converter 2 furnace body 3 bottom blowing nozzle 4 Steel tap 5 booth 6 Lance 7 Secondary material hopper 8 Crude molten steel 9 Converter slag 10 Slag after adding calcium carbonate 11 Porous plug 12 ladle 13 calcium carbonate 14 Storage hopper 15 Refractory lining 16 joints 17a, 17b Sealed iron plate 18 holes 19 ring 20 Ladle iron skin 21 Argon gas pipe 22 Seal gas pipe

─────────────────────────────────────────────────── ─── Continuation of front page (51) Int.Cl. ⁷ Identification code FI theme code (reference) C21C 7/072 C21C 7/072 Z 7/076 7/076 A (72) Inventor Tomoyuki Imamura Kitakyushu City, Fukuoka Prefecture No. 1-1 Tobata-cho, Tobata-ku, Yawata Works, New Steel Co., Ltd. (72) Inventor Noboru Takatsuka No. 1-1, Tobata-cho, Tobata-ku, Kitakyushu, Fukuoka Prefecture (72), Yawata Works, Nippon Steel Co., Ltd. (72) Inventor Ryuzo Hayakawa 1-1 Hibatacho, Tobata-ku, Kitakyushu, Fukuoka Prefecture Inside the Nippon Steel Co., Ltd. Yawata Works (72) Inventor Ryo Watanabe 1-1, Hibahata-cho, Tobata-ku, Kitakyushu, Fukuoka Stock of Nippon Steel F term in Yawata Works (reference) 4K013 AA02 BA02 BA12 CA02 CB09 CF13 DA03 DA08 DA10 DA12 EA01 EA12 EA19 4K070 AA03 AB03 AB07 AB17 AC20 AC23 BD09

Claims (6)

[Claims] 1. A CO ₂ -generating substance is dividedly added after adding a reducing agent to decarburized and smelted chromium-containing steel in a converter.
The chromium oxide in the slag is reduced, and then C
A method for producing a low-nitrogen chromium-containing molten steel, characterized in that the reduced chromium-containing molten steel is tapped in a ladle containing an O ₂ -generating substance, and then depressurized secondary decarburization refining is performed. 2. The method for producing low nitrogen chromium-containing molten steel according to claim 1, wherein a slag-lowering agent is added to the chromium-containing molten steel in the ladle containing the CO ₂ generating substance. Method for producing low nitrogen chromium-containing molten steel. 3. The low-nitrogen chromium-containing solution according to claim 1 or 2.
In the method for producing steel, the CO₂Generated carbon dioxide
Sium and the slag lowering agent is SiO ₂・ Al₂
O₃Concentration of CaO produced using a flux composed of a system
Against SiO₂And Al₂O₃The concentration ratio is 1.3-1.
Low nitrogen-containing black characterized by adjusting to 8
Method for producing molten steel. 4. The method for producing a low nitrogen chromium-containing molten steel according to claim 1, wherein an Al alloy for heating is added to the chromium-containing molten steel in the reduced pressure secondary decarburization refining. A method for producing a low-nitrogen chromium-containing molten steel, characterized by: 5. The method for producing a low nitrogen chromium-containing molten steel according to claim 1, wherein the CO ₂ generating substance is calcium carbonate, and the CO ₂ generating substance added in the reduced pressure secondary decarburization refining is used. CaO and A produced by reaction with Al alloy for heat
A method for producing a low nitrogen chromium-containing molten steel, which comprises adjusting the ratio of l ₂ O ₃ to be 1.2 to 1.6. 6. The method for producing low nitrogen chromium-containing molten steel according to claim 1, wherein the outer periphery of the porous plug provided at the bottom of the ladle is sealed with an argon gas atmosphere. A method for producing a low-nitrogen chromium-containing molten steel, which is characterized by the above.